1. Field of the Invention
This invention relates to the control of the density of solidified aluminum. More particularly, it relates to an improved method for achieving the desired density control.
2. Description of the Prior Art
When aluminum or aluminum alloys are being refined, it is generally desirable to reduce the dissolved hydrogen content to a low level. When a spinning nozzle is employed in the refining process, gases such as argon or nitrogen are commonly used as a sparging gas dispersed throughout the aluminum melt. Hydrogen is removed from the melt by desorption into bubbles of the sparging gas, while other non metallic impurities in the melt are lifted into a dross layer by flotation. The refining operation is continued until the hydrogen content and the particulate content of the melt are reduced to desirably low levels. In practical commercial operations, the refining operation as carried out in order to reduce the particulate content to a desirable low level may actually serve to also reduce the hydrogen content not only to its desirable low level, but to even lower levels in the absence of precautions to assure against such a result. When aluminum is being refined for direct casting into ingots, such further reduction of the hydrogen content is acceptable and has no adverse consequences. When aluminum or aluminum alloys are being refined for casting into molds for the making of parts, however, the reduction of the hydrogen content to a very low level may result in undesirable part shrinkage.
Such cast part shrinkage can be avoided by the presence of hydrogen in the melt. As the melt solidifies, the evolution of fine hydrogen bubbles tends to offset the normal shrinkage that occurs upon solidification. The hydrogen level in the melt, however, must be maintained within certain limits in order to assure that high quality castings are produced. If the hydrogen level is too low, shrinkage will occur. If, on the other hand, said hydrogen level is too high, excessive porosity will exist in the cast part upon the solidification thereof.
In prior practice, recognition has been given to the need, in instances such as those described above with respect to the casting of aluminum into molds, to control the hydrogen content of the melt to be solidified. One approach that was attempted involved the adding of such hydrocarbons as potatoes to the melt, with the results being very erratic and uncontrolled. Attempts to add hydrogen gas by the bubbling of such gas through a pipe into the melt have proved inefficient and difficult to control because of the large size bubbles formed.
A need thus exists for the development of a method for controlling the hydrogen content of an aluminum melt, and hence the density of the aluminum product solidified therefrom. The ability to obtain such control in a desirably short period of time is an important aspect of this development for effective use in practical commercial operations.
It is an object of the invention, therefore, to provide an improved method for the control of the density of solidified aluminum.
It is another object of the invention to provide an improved method for controlling the density of an aluminum product through the control of the hydrogen content of the melt from which aluminum or aluminum alloys are solidified.
It is a further object of the invention to provide a method for minimizing the time required for achieving a desired hydrogen content level in an aluminum melt on a repeatable basis.
With these and other objects in mind, the subject invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.